bcachefs: Fix kmalloc bug in __snapshot_t_mut

When allocating too huge a snapshot table, we should fail gracefully
in __snapshot_t_mut() instead of fail in kmalloc().

Reported-by:  <syzbot+770e99b65e26fa023ab1@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=770e99b65e26fa023ab1
Tested-by:  <syzbot+770e99b65e26fa023ab1@syzkaller.appspotmail.com>
Signed-off-by: Pei Li <peili.dev@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>